The present invention relates to a process for the production of castings with well-defined reproduction of detail and a high accuracy of dimensional tolerance using the gypsum-mold process, as well as to the precision casting produced with the molding process.
Gypsum-mold processes have been well known for a long time. The mold material in this process consists mainly of gypsum. Further constituents can be, for example, sand, cement, sodium silicate, or asbestos. In such mixtures, gypsum has a function of a bonding agent and is also usable in the higher temperature ranges. Precision castings, however, cannot be produced by this process, nor indeed by the vacuum mold process.
A gypsum-mold process is described in Giesserei Lexicon (in translation, The Foundry Lexicon), 1974 (pages 279 to 280). The mold sections or pieces made of gypsum produced in this way are dried in the oven at 105.degree., and are completely unsuitable for the production of metallic precision castings at high temperatures. The gypsum is present in these mold pieces as a dihydrate. With the casting of the molten metal mass, the water escapes so quickly, predominantly in the form of steam, that it causes the mold to split. Gypsum-molds dried at 105.degree. C. are suitable only for castings at room temperature.
U.S. Pat. No. 3,825,058 discloses a process for preparing a mold by a vacuum sealed molding process in which a particulate material, for example, sand, is placed inside a molding box, subjected to a vacuum and compressed to form a mold. The mold has a casting cavity which is defined by a shield member, impermeable to gas, made of synthetic material (plastic) or metal foil. In order to prevent breakdown of the mold during pouring of a molten metal, a tubular member forming a passage serving as a communication means between the mold cavity and the atmosphere is connected to the uppermost portion of the shield member so that the atmospheric pressure can be imparted to the cavity even when the molten metal is being poured into the cavity. In this way, the vacuum in the space between the particles is certain.
Sand-mold processes also have been well known for a long time. Statistics show that the majority of casting series are produced by sand castings. The casting surfaces of the products conditioned by the molding substances employed in the sand cavity necessarily require a high expense for after-treatment, which is quite cost intensive, and today is no longer considered justifiable.